1. Field of the Invention
The present invention relates to antennas, and more particularly to a type of base station antenna that can be easily assembled and disassembled for transportation to different sites and retuned to frequency on location.
2. Description of the Related Art
Base station antennas are usually mounted on the sides of buildings, on short towers, or on tripods. The radiating section of the antenna is open to the air and is made from sections of telescoping aluminum tubing, using hose clamps to fix the sections in place.
These antennas generally fall into three general electrical types:
Type 1: Antennas with a radiating element 1/4 wavelength long or a collinear antenna with a first element 1/4 wavelength long. These antennas are fed at a point of lowest impedance and therefore require a system of radials or counterpoise or a combination of both. Impedance matching is often achieved by slanting the radials at a downward angle or by using a sleeve coaxial with the transmission line to effect a dipole antenna. This type of antenna has the lowest effective gain of the three types described here. PA1 Type 2: Antennas with a radiating element made antiresonant, or about 1/2 wavelength long, or a collinear antenna with an antiresonant first element. These antennas are fed at a point of highest impedance and require no radials or counterpoise system. An "L" network is used to match the feedline impedance to the transmission line. A fringing capacitance between the antenna and the base forms the capacitor. The inductor part of the "L" network is usually external to the antenna so it can be adjusted for matching. This antenna has a gain of several dB over the Type 1 antennas described above. PA1 Type 3: Antennas with a radiating element made about 5/8 wavelength long so that the real part of the input impedance is 50 ohms and the reactive part is capacitive or, if collinear, tuned in the same fashion. These antennas may be matched by inserting a series inductance which tunes out the capacitive portion of the reactance and requires a ground plane as with Type 1 antennas. These antennas have a slight gain improvement over Type 2.
The requirement for a ground plane, with its need for mounting area and the tendency of these antennas to get bent, makes Type 1 and 3 antennas unattractive for temporary installations. The 1/2-wavelength antennas can be matched to a transmission line with an inductor outside the antenna housing so it can be easily adjusted by the user. This may also be done with the network inside the mounting sleeve if the antenna is fixed tuned, and the frequency bandwidth of an antenna may be increased by adding a compensating network.
Typical commercial versions of Type 2 antennas, implementing the application of radials and/or sleeves for counterpoises, have a typical "L" network made from the transmission line and enclosed inside a sleeve used to mount an antenna.
Some Type 2 antennas use a "ring"-style matching network and feedline connection exposed to the weather. The "L" matching network uses the fringe capacitance between the antenna radiator and the mounting sleeve. Others use an "L" network, a shunt capacitor, and a series inductor to match a high impedance to a 50 ohm impedance.
There are also antennas that replace the radials on a 1/4-wavelength antenna with a sleeve called a "sperrtopf." This sleeve is 1/4 wavelength long at the antenna's operating frequency and may be reversed. Its purpose is to choke current from the coax of the feedline. Such antennas, often used as base station antennas with internal matching networks, use mounting sleeves that are 1/4 wavelength long.
There are antennas which use an internal matching network assembled inside a 1/4-wavelength tube, or sleeve. A tap on the transmission line acts as a capacitor, and the transmission line having a Z.sub.0 somewhat higher than 50 ohms replaces the inductor. The sleeve also acts as an RF (radio frequency) choke, keeping RF current off the feedline.
U.S. Pat. No. 4,835,539 issued to Paschen on May 30, 1989, for a "Broadbanded Microstrip Antenna Having Series-Broadbanding Capacitance Integral with Feedline Connection," describes a broad-banding network.